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With respect to IFR alternate minimums, why are helicopter minimums lower than airplane minimums?
According to 91.169, airplanes need 2,000-ft elevation while helicopters need 1,000 ft. Airplanes also need three set miles visibility while helicopters only need two. Is this because helicopters fly lower and slower than airplanes?

@757toga sorry for the late response. I assume there's a mix up with 91.169 part B and part C. My original question was referring to part B. But it seems that in both scenarios helicopters have the lower minimums. I was just wondering if that anything to do with the fact that helicopters fly lower and slower than airplanes. Sorry again for the mix up. I greatly appreciate your time spent researching.

Yes. The Helicopter can slow down enough to better avoid obstacles, even stop if it's within its Out Of Ground Effect hover ceiling, so credit for this is taken in defining minimum visibility conditions.

How timely, I came across this video the other day.https://youtu.be/RxpYTByTHB8?t=499


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At what height would a pilot apply TOGA during a go around Vs getting simply vectored somewhere else?
If a pilot has to go around Early in a approach, he can simply be vectored to get reestablish. At what point would the pilot decide to apply TOGA other than obviously at minimums?

Anytime I want to execute the missed approach I would push the TOGA button anywhere after crossing the final approach fix.  I don't know what specific airplane you fly but the ones I do pushing the TOGA button does these things:

Disconnect autopilot
Sets flight director to 9° nose high in a wings level condition
Sequences the FMS (FMC) to fly the missed approach

That's how the CRJs work, which have the Collins Pro Line Flight Director.

When operating into airports with parallel runways airliners are sometimes given a runway change after being established on one ILS.
The procedure to follow is an “ILS Breakout”. eg (B767) If done with the autopilot engaged this requires a go-around to be commenced, initially TOGA, followed by a sequence of selections to hold the current altitude and control the airspeed before other selections to recommence a descent and to intercept the ILS for the new runway.
In the B767 the autopilot stays on when TOGA is selected.
A very busy procedure that should not be attempted in IMC below 1500 ft. ie Go-around and follow the missed approach procedure for the current runway.

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Why does induced downwash vary across a wing in a sideslip?
in this excellent answer, it states this :

A well-known effect of wing sweep is the variation of induced downwash along the span from the trailing wake that produces an additional lift distribution characterized by increased loading on the aft wing and reduced additional lift on the forward wing. For a wing with no twist or bend, this results in a significant rolling moment, tending to roll the forward wing downward. There is also a yawing moment from the asymmetrical distribution of induced drag that tends to unsweep the wing.

My question is, what makes the induced downwash along the span vary? Also, when it says "trailing wake", does that mean the wake turbulence from the aircraft, and if so how would that interfere with the induced downwash? (the 2 big vortices formed by the aircraft is what I mean by wake turbulence)

A bit unrelated, but I was thinking about this also, so I’ll put it here. Say your left wing for whatever reason has no tip vortex. I always thought that would make you roll to the right, as the downwash from the vortices would be “pulling” the right wing down. Is that correct?

Good question.  Wouldn't the absence of the tip vortex indicate the left wing was producing no lift? As indicated by the downwash, isn't the net displacement of momentum downward on the right wing?  In that case, with lift indicated on the right wing, wouldn't the aircraft roll to the left?

Yes that would definitely indicate that, but pretend that the lift is still being created but somehow the tip vortex isn’t. Isn’t possible in real life, but just a thought experiment.

@ThomasPerry Also, did you mean downwash from the tip vortices (induced downwash) or just the plain downwash from the wing?

Conceptually, downwash and the presence of tip vortices are all part of one continuous process.

Related: https://aviation.stackexchange.com/questions/104365/what-happens-to-tip-vortices-in-a-sideslip? -- see all answers

This effect is really only substantial for swept wings.
Here is the lift distribution on an un-swept rectangular wing at 10deg AoA at 0 and 10 deg Beta.

Notice the general slight reduction in lift - but also the slight asymmetry introduced.
If you were to plot the wakes, they are pretty uninteresting.  The 10deg Beta wake is essentially sheared 10deg to mostly align with the freestream.
However, let's look at the same wing with 20deg of sweep.  If you draw some velocity diagrams, you'll find that the left and right wing seem to end up seeing substantially different angle of attack.

It is clear that the two half-wings are seeing very different flow situations.  This results in a dramatically changed lift distribution for the two.

Notice the large change in lift distribution on the two wing halves.  It is as if the right wing sees a higher angle of attack than the left wing.
The wake is the sheet of vorticity that trails behind a lifting surface.  It is stronger at the wingtips than across the rest of the wing, but it exists everywhere there is circulation.
Edits:
It looks like I opened a can of worms with the angle of attack analogy.
I based my comment on foggy memories of a classical argument for a $C_{l,\Beta}$ derivative (dihedral effect, rolling moment wrt. sideslip) and how it depends on wing sweep angle.
For example, from: Etkin B. and Reid, L.D., "Dynamics of Flight; Stability and Control", Third Edition. John Wiley & Sons 1996.

quiet flyer -- My memory was wrong -- they make an argument based on velocity magnitude perpendicular to the wing -- not based on velocity triangles changing the local angle of attack.  I think this tracks better with your understanding.
Interestingly, the dihedral effect due to dihedral is calculated with the velocity triangles I remember.
Amusingly, he goes on to make an argument about the oblique wing in this discussion...

Sophit - you're right, I read the OP's question without clicking on the link.  So I was answering for actual swept wings.
For Robert DiGiovanni, Here is a set of load distributions for the straight wing at 0,30 deg Beta and 5,10 Alpha.

And the 30deg Beta, 10deg Alpha plot of delta Cp and the wake -- rotated about 30 degrees to make it look like an oblique wing.

Not sure if I’m not picking up on something, but is the reason the 2 half’s of the wings end up at different AoA because of the tip vortices?

Please note that in the linked answer, with "swept wing" they actually mean "oblique" wing... The present answer, albeit being technically perfect, is therefore formally wrong.

@RobMcDonald please run the unswept wing at 30 degrees beta and also try 5 degrees AoA (to continue work on oblique wings).  The swept case reveals that a yaw unsweeps one and sweeps the other more (good for Dutch Roll studies).  I am curious to see if the effect of sweep on lift is linear, especially at higher beta.

Seperate question: in the view from above of the swept wing, if the airflow is crossing from right to left in the aircraft's reference frame, why do the airflow lines right at each wingtip seemed to be kinked the opposite way?  I.e. on the right wingtip the line bends slightly outboard but on the left wingtip it bends slightly inboard?  Is that because the right wing is generating more lift than the left wing?

I'll accept "go to chat" at the first opportunity and delete most of these comments.  What it really all boils down to is this-- we know that sweeping a wing tends to reduce its max Cl and generally flattens out the Cl vs aoa curve.  (There's a figure showing this in the book Aerodynamics for Naval Aviators.)  In the sideslip case, one wing is much more swept than the other, relative to the airflow.   So it's no surprise that it's lift coefficient is reduced.  Is the reduction in lift coefficient due to sweep specifically attributable to greater induced downwash?

Food for thought-- this related answer https://aviation.stackexchange.com/a/9292/34686 does state that sweep causes the "effective angle of attack" to vary between the two wings in a sideslip.  I guess "effective angle of attack" is measured with one edge the protractor normal to the quarter-chord line?  I'll have to think about that a bit.  I'm not sure it's really fair to list points 1 and 2 at the front of that other answer as 2 different phenomena, aren't they 2 different ways of stating the same thing?

(Ctd) Anyway in that answer by "effective a-o-a" it's pretty clear he's defining a-o-a relative to the free-stream flow, i.e. not considering induced downwash.  The answer doesn't seem to consider induced downwash, or differences in induced downwash between the two wings, at all.  And yet he still comes to the conclusion that sweep tends to produce a roll torque during sideslip (at least at non-zero a-o-a).

(Ctd) No surprise there, but I guess I'm circling back to my question of whether the lift difference between the two (swept) wings highlighted in your answer can really be attributed primarily or largely to differences in induced downwash and other effects related to displacement of the tip vortex/ wake vortex systems, or not.

I would agree that points 1 and 2 in the other post are not different phenemona.  When you take the simple sweep theory approach of 'normal velocity matters', you can book-keep the change as a change in chord, velocity, alpha, or whatever.  If the whole expression is multiplied by $\cos(\Lambda)$, then you can choose to interpret that effect on any single term.  I would argue that this is a fundamental geometric effect -- it is not an effect of induced downwash.

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Why does turbulence over a wing make drag?
Drag is the result of air ‘pushing’ the opposite direction of flight, if you want to think of it that way. The skin friction drag is made from the air trying to pull the wing opposite of the flight direction. My question is, why would turbulent flow change this?
Say you somehow have air on a wing that is going exactly 90 degrees to the direction of flight. Would this create drag for the wing, acting opposite of the flight direction? I would think not because the air is pushing the wing sideways not backward.
Basically turbulent air is air moving in random directions. So say it randomly moves left (spanwise) on a wing. Why would that make drag? Or if it moved right?

Stop thinking about the direction of the velocity as a force pushing on a wing.  Velocity is not a force.  Stop it, you're making yourself confused.

Turbulence has different meanings.

Normal people (not engineers) think of turbulence as 'rough air' that an airplane encounters in flight.  It can cause the aircraft to bump around violently and is scary and potentially dangerous.
This kind of turbulence is caused by updrafts and downdrafts from thermal activity, mountain ridges, and other weather and terrain issues.  In a small aircraft, you can also encounter turbulence caused by a larger aircraft in front of you.
Engineers also think about turbulent flow vs. laminar flow.  Here, turbulence is microscopic tiny random oscillations in the flow.  They do not have a predominant net direction that can cause the aircraft to heave or jerk.
A turbulent boundary layer has fundamentally different characteristics from a laminar boundary layer.
It is almost like the fluid itself changes -- from air to water (not really, but the important part is that properties of the flow change and the differences we see is due to the changes in those properties.  The changes are not due to some bulk velocity or major thing).
Of first importance, at the same Reynolds number, the skin friction coefficient of a turbulent boundary layer is higher than of a laminar boundary layer.  Note this is on a log scale.

At Re=5e5, the turbulent skin friction coefficient is about 3x higher than laminar.
There are other situations where turbulent flow is good.
Turbulent flow also has a much higher heat transfer coefficient than laminar flow.  So, if you're designing a heat sink or radiator to pull heat out of something, you would much rather have turbulent flow.
Also, turbulent flow is less likely to separate than laminar flow.  So a golf ball uses dimples to force transition to turbulence and therefore keep the flow attached to reduce the separation drag on a golf ball and allow it to fly much further.  Separation drag is almost always a bigger deal than skin friction drag.  Only worry about skin friction drag after you've taken care of separation drag.

I see. So is there an acknowledged reason why the properties of turbulent flow change so much, like from air to water?

Sorry if I missed something obvious.

Recall how we've talked about how a gas is a bunch of random particles bouncing around.  That behavior helps establish certain properties of the gas (perfect gas law P=rho R T, speed of sound a=sqrt(gamma R T), etc.).  However, at larger length scales, we don't really need to worry about the tiny particles, we instead treat the air as a continuum.  Laminar vs. turbulence is a similar thing, but at an intermediate length scale.  Large compared to the size of molecules, small compared to the size of airplanes.

At the length scale of laminar vs. turbulent flow, we either have nice orderly flow that is all parallel to the velocity of the net flow.  Or, we have random little eddies that are chaotic and random -- but, their average velocity is still going with the flow.  Imagine busy highway traffic -- on the one hand, you have everyone staying in their lane, going the same speed, generally well behaved.  On the other hand, you have cars all going random speeds, changing lanes randomly, total chaos -- but the average traffic flow is the same.

I think the change in heat transfer is the easiest to understand.  In laminar flow, the wall heats the first layer, but each orderly layer must heat the next layer.  Air is a pretty good insulator, so there is only so much heat that can get pulled out this way.  However, in turbulent flow, particles near the body pick up heat, but then they move away from the body, where they transfer heat to other particles, or mix with those particles.  Because of this, turbulent flow is a much better conductor of heat.

Ah okay, thanks. So using your example, the cars would be very random, different speeds, just random, but the traffic flow is still the same. How would this lead to increased drag?

Imagine a bowl of water that has been sitting still for a long time.  Then you put one drop of food coloring into it.  Watch how the color spreads in the water.  This is called diffusion.  It is a random mixing and spread of particles with no net motion.  This is also how smells travel across a room.  Now compare the spread of the dye to a bowl that is being vigorously stirred.  Now imagine that stirring is a bunch of tiny random motions.  It won't surprise you that diffusion happens much faster in turbulent flow than laminar flow.

Yeah it definitely makes sense how the air would diffuse better, because of the random motion of the molecules, but I can't see how that would increase drag in any way.

Each of these properties -- friction coefficient, heat transfer, and diffusion -- are all dependent on laminar vs. turbulent -- and we treat them as bulk properties of the flow (ignoring the tiny random eddies) and we just track the bulk velocity.

Oh okay, good to know. So is it from those effects combined (diffusion, heat transfer, friction) that makes turbulent flow have more drag? Another way to ask it is does the bulk velocity increase with turbulent flow?

I'm not sure that my traffic analogy will help explain the difference in skin friction coefficient.  I'm also not sure that I have an intuitive explanation that is also rigorous (correct) about why skin friction of a turbulent flow is higher than a laminar flow.  At a deep level, it turns out that heat transfer and skin friction and diffusion are all actually very closely related -- and we can actually use measurements of heat transfer to predict skin friction and vis. versa (and with diffusion).

OK here https://en.wikipedia.org/wiki/Transport_phenomena is a good discussion of the sameness of friction, heat transfer, and diffusion.  They are all governed by the diffusion equation (and diffusion processes) -- friction in a gas is governed by the transfer of momentum -- mass diffusion is the transfer of mass -- and heat transfer is the transfer of heat.  Since they are all diffusion processes, it makes sense that they are all sensitive to turbulent vs. laminar flow.

Ah okay I think I see where you're coming from. So from my own research and the research you provided, this is what I think so far. So the surface of a plane wing makes friction and slows the air down. This slows the air above it down, and it goes on. With turbulent flow, this effect is exaggerated because the flow mixes more, and the wing is required to pull more air along. Is this correct?

Short answer:
turbulent flow causes vigorous mixing of the flow nearest the wing with flow further away from the wing. this tends to share the energy imparted from the wing to the airflow nearest the wing with airflow further away from the wing. This speeds up the air further from the wing while slowing down the airflow nearest the wing and this effect increases the drag asserted on the wing by the airflow nearest it.

Ah okay, thanks a lot! This is exactly what I was thinking happened.

Yes, this is the right simple explanation. The turbulent mixing brings much more bigger momentum fluxes from the wing surface.

Vladimir F, are you a slavic hero?

Let's slightly change your concept and reframe your thinking, if that is ok.  Think of drag as the wing dragging air in the direction of flight.  Now think of the process by which that would happen.  Momentum from the aircraft is imparted to that air.  For the air that has passed over the wing, we can actually measure that momentum.
Ok... Now let's rewind for just a second and look at the drag associated with flight.  To lift the aircraft, we have to expend energy, develop power, and generate thrust.  This drag is associated with lift.  Just set that aside; we are not talking about that kind of drag.
We are talking about drag associated with the boundary layer that develops in the flow of air over the surface of the wing.  For the ideal conditions generally discussed in conceptualizing flight, we think of the aircraft passing through its parcel of air.  There is no wind, this air is still.  Consequently, the wing is passing through still air.  The turbulence produced by the wing passing through that still air does not move anywhere except by the momentum imparted to that air through the boundary layer on the wing of the aircraft passing through that air. In producing that turbulence, energy is transferred into the boundary layer through drag consequent to, or from the aircraft passing through that air.  That drag happens through skin friction resulting from the movement of the turbulent flow over the surface of the wing, itself. Momentum is imparted to that turbulent flow, mostly as motion of the turbulent wake in the direction of the aircraft, but also by momentum in the sustained turbulence that is left behind.  Let's think about this for a moment...
That turbulent boundary layer moves downstream over the surface of the wing, and through skin friction, energy is imparted into, and in development of, that turbulent boundary layer.  Consequently, in that still air, we can actually see the movement of the turbulence that remains, as the turbulent wake left behind the wing moves gently in the direction of flight of the wing, itself.  If we know the properties of that air, and the width and average velocity of the flow in the wake (remember, it has general velocity in the direction of flight), then we can determine the momentum of that air, and the power expended in producing its motion.
A common point of confusion is the conception that turbulence is the random motion of air moving in random directions.  For the wing in flight, airflow over the wing, whether laminar or turbulent, moves in a direction opposite the direction of flight.  There is no net motion to the left or right; all of the motion in a relative sense is downstream over the surface of the wing.$^1$  If we view the wing as passing through still air, the turbulence left behind in the wake is truly of random motion.  Careful observation of the air in that wake will show general motion in the direction of flight, that motion determined by the energy imparted to it through momentum lost from the aircraft, itself. This can be measured.
How do we make these measurements?  If our wing is in a wind tunnel, we can attach a scale to the wing and measure the force of drag.  We can put a velocity-survey rake in the wake behind the wing and measure the velocity of the airflow in that wake.  We can even make this measurement on a wing in flight.  Nevertheless, you may say, the wing in a wind tunnel is still and the air is moving.  No matter, conceptually everything is essentially the same as the wing in flight through still air.
Now let's consider something that seems totally strange yet is perfectly natural - an aspect of practice in aircraft and wing design, a fundamental principal that is applied in wing design.  In this case, we consider the wing as still and air as moving over the wing surface.  Turbulence forms in an adverse pressure gradient. This is a physical fact that cannot be changed.  What we can change is the drag.
The shape of the wing section can be adjusted so that air flowing over the surface through this adverse pressure gradient is more gently slowed.  The consequence is to thicken the boundary layer and reduce skin friction.  Further, less motion is imparted to the turbulent wake as the wing drags this flow in the direction of flight. This process can be determined in producing a turbulent boundary layer that has no skin friction whatsoever.  B S Stratford was the first to demonstrate this more than 70 years ago in a wind-tunnel experiment where the shape and concavity of one wall of the tunnel could adjusted to produce a pressure gradient resulting in a turbulent boundary layer having zero skin friction.  As mentioned elsewhere, my term for this is infinitely separating...

$^1$ In this case, as noted previously, we are not discussing lift induced flow which may be skewed to the left or right over the surface of the wing.

I make my living by simulating turbulence and teaching about it (teaching mostly in atmosphere, not mechanical engineering) and I have hard time understanding these paragraphs. If they are supposed to be understandable by a layperson, then these persons may have some hard time as well.

@VladimirFГероямслава Noted, and also exactly understood. Taken to heart and will look into text modifications, simpler explanation, and examples (as time allows).

One simple explanation is just math, though it is possible that this is already accounted for as part of what makes laminar flow a different regime.
You mentioned air moving in random directions, but that doesn't actually cancel out when you're moving against friction, because of the velocity squared term.  If you're going 10 units on average, that makes for a factor of 100.  But if one section is moving at 9 and another at 11, that makes for (121+81)/2 = 101 units of effective drag.

authors: Vladimir F Героям слава


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What happens when shockwaves interact?
As seen here, there are two T-38's going supersonic. What happens when those shockwaves interact? They seem to dissipate in some places on this photo when they interact. Any source online says that they will "produce complex patterns of compression, rarefaction, and reflection."
However I can't find anything on why that happens.

Remember, shockwaves and expansion fans are all about turning the flow.  The required turning can be observed from the boundary conditions (the walls).
Each shock / fan exists to turn the flow from one condition to the next.
Where two flow features meet, they either add together or cancel out -- depending on whether their combined turning is complimentary or canceling.

oh okay, so generally they don't reflect or refract?

I would think they do because if you look at a lambda shock, the normal and oblique shock seem to reflect off of each other.

There are many possible situations that can happen.  They are categorized as Type 1, 2, 3, 4 etc.  Check out this reference https://www.osti.gov/servlets/purl/4480948 jump straight to the pictures around page 86.  Note, the flow is right-to-left not left-to-right (which I'm more used to).


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What does air becoming compressible change?
You always hear that around Mach 0.3, air technically becomes compressible. What actually changes when this happens, in subsonic and supersonic flow?
Does it affect the formation of shocks?

Every macroscopic substance is compressible.  Perhaps you are asking if the compression is relevant to aerodynamics?

Air does not suddenly become compressible.  It is a gradual change that can be delayed or accelerated by a number of factors.  Mach 0.3 to 0.6 is a good range for most airplane related things.
We usually treat air as a continuum -- it is of course actually made up of particles bouncing off of one another.  We can derive the equation of state $P=\rho\,R\,T$ by considering particles bouncing around in a box.  The velocity of the particles is related to the temperature.  The density is related to the mass of the particles and how many are in the box.  The pressure is the average force caused by the particles hitting the walls of the box.
From this same argument, we can derive the equation for the speed of sound $a=\sqrt{\gamma\,R\,T}$.  The speed of sound is the speed of a pressure wave in a gas.  It is related to how fast those colliding particles are running into one another -- those collisions propagate the wave at the molecular level.  We care about it at the continuum level.
Pressure waves are how disturbances are communicated in a fluid.  If you put a blockage in a flow -- how is the existence of that blockage communicated upstream and downstream?  How much time does that communication take?
The Mach number is the ratio of the flow's characteristic speed (usually the freestream) to the speed of sound.  I.e. how fast is the flow moving vs. how fast does information propagate.
In the limit of incompressible flow (M=0), information propagates infinitely faster than the flow.  I.e. information travels everywhere instantly.  A change in a flow is felt everywhere instantly.
Incompressible flow is the condition where the time for information flow is not important to the flow (because the flow is slow).
Compressible flow is the condition where the time for information flow is important to the flow.
This is actually represented by a change in the differential equations for the flow.  Incompressible flow is modeled with an elliptical differential equation -- everything is felt everywhere instantly.  Compressible flow is modeled with a hyperbolic differential equation -- propagation time matters.
We call this incompressible vs. compressible because thinking back to our box of particles -- if the flow is slow compared to the collisions, then the particles can always spread themselves out equally -- the density will be uniform, and the flow will not compress.  However, if the flow is fast compared to the collisions, then the particles can bunch up.  They can compress in certain areas relative to others, the density of the flow can be non-constant.
If a disturbance is small (say a very thin airfoil), then the effects of compressibility (how much information travel time matters) may not become important until M=0.8.
If a disturbance is large (a cylinder in the flow), then the effects of compressibility will become important earlier.
Shocks happen because the flow is faster than the disturbance -- so the flow can't gradually react to a disturbance.  Instead, it must find some way to abruptly react.  This abrupt reaction is a shock.
Let us change gears for a moment....  If you put your thumb across the opening to a garden hose, the flow speeds up and the spray out of the hose goes further.  Your thumb is the blockage.  The water is incompressible.  It must speed up to get by your thumb.
Imagine you are in heavy traffic on a four-lane highway.  Up ahead, the highway narrows down to two lanes.  Traffic slows down and everyone goes through the blockage very slowly.
How are these two situations different?
In traffic, information about the existence of the blockage travels slowly.  You notice the car in front of you slowing down (brake lights).  You have some reaction time before you press your brakes.  This human reaction time dictates the speed that information propagates through traffic.  As you get close to the blockage, the space between cars is reduced (compressibility), and everything slows down to some fixed rate (choked flow).  It should not surprise you to learn that traffic flow is modeled with a hyperbolic differential equation.
What if traffic were modeled with an elliptical differential equation?  This would require communication of information between cars to be instant.  Let's say they were all robot cars communicating through a coordinated computer network (no humans and brake lights and reaction times).  If a highway full of instant communicating and reacting cars approached a blockage, they would (instead of slowing down to a crawl), accelerate to 2x their speed and merge perfectly together to make it through the blockage -- just like water through the garden hose.

I see. Sorry for the late comment, but I had a small question after coming back to this. So if air approaches a plate which is facing the wind, when the air gets to it, it will compress given it’s at high enough speed. The reason is because the air didn’t have enough time to react, like you said. So if that air compresses, will that compressed air expand in all directions? Imagine the same scenario but with the leading edge of a wing. Would the air also expand spanwise and in the other directions, because it got compressed, and it is expanding?

Air does not collide with the object and then compress like a spring.  Aerodynamics is not something we solve at a point -- it happens everywhere around a body.  We must solve for what happens everywhere simultaneously.  When flow is fast enough to be considered compressible, then there can be significant density variations across the field -- density is not constant.  In your example of a wing, we will expect density to vary around the wing, but it is a field variable as each particle flows, its density will either increase or decrease along the streamline that it follows.

I see. Last question, because I’m not going to keep asking small follow up questions. That makes sense that the air won’t compress at one point. So say you were to follow a chunk or cube of air heading towards the leading edge. The flow is compressible here, so the cube of air starts to compress when it gets near the LE. This means higher density, and usually higher density means higher pressure. Knowing that, wouldn’t that high pressure cube of air expand upwards and in all directions? In other words, why would the density only change along the streamline that it follows, like you said?

There is another streamline just above it.  That streamline has its own problems to deal with.  It is compressing a little bit too.  Its pressure is changing too.  So is the one to the left and the one to the right.  This is what I mean by it being a field solution.  All the streamlines have to deal with their own problems.  There isn't just one streamline surrounded by vacuum or by ambient air.  They're all in the situation together.


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Why are supercritical airfoils not used in GA aircraft?
Why are supercritical airfoils not used in GA aircraft? The 787 is assumed to use a supercritical airfoil, so why don't GA aircraft use them?
My guess is that they're harder to manufacture, and a lot of GA aircraft today are older.
(Not sure though)
(I'm mainly referring to smaller propeller planes when I say GA)

Actually, Whitcomb's GA airfoils were based on experience gained with supercritical airfoils. They share the large leading edge radius and the strong rear loading with them.
Other than that, suppressing shocks is not needed when flying at speeds where air is almost incompressible.

Because supercritical airfoils are designed for the transonic regime of flight speeds, which GA aircraft do not operate in. Many other considerations, such as flutter, limit GA aircraft speeds even in a dive. For GA aircraft, engines, airframe, and yes, even wings, are designed to only operate in a far slower speed range than a 787.
Summary: supercritical = fast and GA = slow, so supercritical ≠ GA.

oh okay, but don't supercritical airfoils still reduce drag at lower speeds?

If they do, it is because they sometimes achieve some extent of laminar flow -- however, at low speed, you're better off using a laminar flow airfoil if that is what you want.

Supercritical airfoils are like swept wings in that they are used to avoid the effect of compressibility, which creates problems not present at lower speeds and which imposes an additional constraint on design choices. When one says that they reduce drag, that is compared to a wing optimized for lower speed being used at a speed where compressibility is a problem, not that they reduce drag in all circumstances. Not only does this give no reason to think they are better in other cases, the fact that they are a response to an additional constraint suggests that they are likely to be worse.

re: sdenham's good comment, this "likely to be worse" is even likelier when considering that "worse" may be due to other factors such as manufacturability (e.g., developable surface that can be formed from sheet material) and resulting structural mass (e.g., skin thickness needed to maintain desired profile despite surface concavity) in addition to strictly aerodynamic performance of the resulting surface. Even ease of analysis, ease of modification, or response to bug and ice accumulation can be considerations that affect the "best" result in the real world.

The Piper Tomahawk, Beech BE.77 and the Dornier Do228 all used the GAW-1 airfoil, which is shaped like the higher speed supercritical airfoils. The certified OMF Symphony SA160 (formerly kit built GlaStar) uses the similar GAW-2. Features in this usage are the thick airfoil section making for a lighter structure, rounded leading edge which allows high cl at high angles of attack and a wide drag bucket for efficiency. The experimental X57 Maxwell also uses this airfoil.

Well, the GA(W) airfoils have markedly more camber at mid-chord. They are no longer suitable for transsonic flight.


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What is the main reason raked wingtips are more efficient than winglets?
What is the main reason that these:

Are more efficient than these?:

My main guess is that they improve L/D compared to winglets, as they provide more upwards lift making them more efficient.
It also would make sense if they reduced tip vortices because of the sweep and their shape reducing pressure differential, (more so than winglets).

Does this answer your question? Plus all the links in that answer.

More span is always better than winglets.  Winglets become beneficial when you are in a span constrained environment.
Any time you try to compare something in aircraft design (or anything else really), you have to really understand what you are holding constant and what you are varying.
It is nearly impossible to take two arbitrary features from two aircraft and make a direct comparison of which is better / worse -- Even though this seems like a similar comparison (the wing tip treatment of two transport category aircraft), the requirements for each aircraft are surely different -- and those differences dictate the design.


text:
Why don’t shocks refract through other shocks?
The picture in this answer shows a shock traveling into the ground. When it does so it refracts.
For this example pretend that if a lower strength shock went through a higher strength shock, there would be no rarefraction.
So the shock traveling into the ground, (higher density than air) refracts.
If this is the case, why wouldn’t a lower strength shock traveling into a higher strength shock make the lower strength shock refract, as it’s going through a higher density shock?
The definition of refraction is :

the fact or phenomenon of light, radio waves, etc. being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density

So I wouldn’t understand why a lower density shock wouldn’t refract through a higher density shock. (I’m assuming the higher strength shock is higher density).

Also, if anyone knows of some good resources to learn about this subject, I’d appreciate if you could share! I can’t seem to find any information about this topic besides a little excerpt from Wikipedia.

I might recommend:  https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/00-80T-80.pdf  It's old, but a classic with lots of good information in it.  Perhaps this will help satisfy your commendable thirst for knowledge!

Why do you think shock refraction is a thing?  Find us citable references that talk about it and give examples and we can discuss it.
The difference in density of air is not enough for it to be considered a different medium.  The difference in speed of sound between gas when it has been shocked is already taken care of in the other kinds of shock interaction you have studied.
Instead, by the definition of refraction you provided, it would be limited to air-liquid and air-solid interfaces.  Unless you concoct some situation where there is a gas-gas interaction and the two gasses are clearly separate and do not mix.
Shock refraction is a very limited concept.  You are trying to apply that word to a much broader range of situations -- or you are asking why it does not apply much more broadly.  You may as well be making something up.  "Why don't shocks coagulate with other shocks?"

Ah okay, thanks for your answer. So isn’t the definition of a shock when there is a rapid change in properties? Wouldn’t that be a situation where there is a gas - gas interaction where they don’t mix? (Where the shock doesn’t mix with surrounding air)

The shock is air.  The shock is not an object or matter separate from anything else.  It is just a location that we identify because properties change quickly.
Yes, slip lines create regions where gas 'doesn't mix' -- but that isn't enough to make it a different medium.
You're looking for a hornless unicorn -- everybody else just calls that a horse.


text:
Why does more lift make a stronger shock?
Why does more lift make an aircraft shock stronger? In this answer, it says more lift will make a stronger shock.

The aircraft's lift contributes to the strength of the boom.

What part of making higher lift will cause a stronger shock?
I tried looking through Aerodynamics for Naval Aviators, but I couldn’t seem to find anything on this, unless I missed something.
Also looked through Introduction to Flight by John Anderson, but found nothing (again unless I wasn’t looking in the right place).

Think about lift from a thin flat plate at supersonic speed.  There are some great notes here.

This is the minimum situation to describe supersonic lift.  From far away, supersonic lift looks like this.
The main shock that will influence the boom is the one in the front from the lower surface.  Clearly, at zero angle of attack, there is not shock -- but there is no lift.
As the angle of attack is increased, so is the required turning angle, and therefore the strength of the shock is increased -- which leads to a louder boom.
You can 'add' the effects of thickness on top of this, but it really mostly adds complication.

I see, that makes sense. Thanks!


text:
Could the folding wingtips of a plane (ex. F-35C) be used as a vertical stabilizer?
In the rare event of the vertical stabilizers being ripped off and the rest of the plane surviving, could folding wingtips (given they are far enough behind the CoM to create a stabilizing moment) be used as a "replacement" for the vertical stabilizers?

Not really folding wingtips but the Rutan VariEze uses that concept.

@sophit - As did the B-70. But those were foldable in flight, by design. https://aviation.stackexchange.com/questions/22745/why-werent-the-xb-70s-wingtips-fixed-in-the-down-position

@WPNSGuy: yes, very good example indeed  Even if in the case of the Valkyrie there was more than just increasing lateral stability.

If the vertical wing tips have aerodynamic centers that are aft of the center of mass, there will be some yaw stabilizing effect by the tips.
Problem is, the effect will be quite minor, the arm being so short, and without vertical stabilizers, the overall aerodynamic center of the entire body is going to be forward of the center of mass, because there is more fuselage area ahead of the center of mass than behind it.
The turned up tips having so little "leverage" they will likely just make the thing somewhat less unstable in yaw, but not positively stable. They are just too far forward to make more than a minor difference.
Remember, it's just a big weathervane pivoting about the center of mass. Look at a side profile without vertical fins, and imagine where the overall center of the finless profile presented is, relative to the center of mass (assume the center of mass is about 1/4 of the wing chord aft of the leading edge). If the total profile footprint is still forward of the center of mass, the weathervane, or airplane, will want to switch ends and point the other way. Then it will be stable in yaw, just going backwards.

Agreed. unless the tips were as far back as the vertical stabilizers, then they would not provide as much stability as the vertical stabilizers for sure.

When you put floats on an airplane, they move the yaw axis neutral point (aerodynamic centre) forward, reducing yaw stability, b/c the floats have more surface area ahead of the CofG than aft. That's why you usually see little finlets or ventral fins added to the tail when floats are installed. On airplanes that DON'T include fins as part of the float kit,you will notice the decrease in yaw stability when floats are installed. Nose wanders in bumps, and adverse yaw with aileron increases.

Well, no. There is no control authority or code to tell the wingtips to act as a replacement for the vertical stab(s) which have been blown off.
Additionally, you can't fold the wingtips up in flight.
Might they act as sort of a yaw stabilizer? Maybe. But not with any steerable control from the cockpit.
If the system had been designed to do this, like the XB-70, then yes. Since it was not, this will not work. Especially if trying to compensate for having the existing vertical stabs "blown off".

Yeah I was thinking more along the lines of yaw stabilizer. Just something to make you not immediately go into a spin.

@Wyatt - Whatever caused the RUD (Rapid Unplanned Disassembly) of the vertical stabs might have an influence on stability.

haha yeah you're probably right. Just was wondering if in a rare case if the wingtips could provide stability.

"you can't fold the wingtips up in flight" - Citation required. Why not?

@Richard - I believe it is controlled by a weight on wheels switch, much like munitions release. No weight on wheels, no fold. Also, only the C model has folding wings. The -A and -B do not.

I feel it is possible that folding the outer panels in flight would have a greater effect on the stability in pitch than in yaw - or in roll, for that matter, supposing that it changes the effective dihedral.

I agree with the diagram by Mins, however, creating a source of drag at the wingtips would create a yaw force in the correct (rearward) direction with ample 'arm'.
The MU-2 uses drag-brakes instead of ailerons to induce both yaw and bank, however, in general, it is frowned upon to induce drag as part of maneuvering as this creates unexpected stalls in low-speed turns. i.e. MU-2.
The wingtip design would not require the wingtips to pivot upright, just remain as-is while inducing drag, such as a split trailing edge as speed brake, just like the Space Shuttle vertical fin did.

More or less what others already said, but with a picture...
Yaw control/stabilization is a rotation around the yaw axis, created by a force acting perpendicularly to its arm (left below).

In your suggestion the vertical tip would create a left/right force (right above), applied with an arm which is parallel (the line between the surface and the center of gravity. This results in:

The force is not amplified (arm parallel to the direction of the force).

The resulting moment tends to translate the aircraft left or right, it has no yaw action.

No mentioning the lack of crew control, the reduced lift of the wing, the strength of the unlocked tips, etc.

ah, I thought that might be the case. I was thinking that sometimes swept wings will have tips aft of the CoM, creating a stabilization effect if the tips fold up.

@Wyatt: You're right on the principle, this is like a twin tail, however an inefficient one, the arm is short and the area of the tip is small, limiting the torque that can be created. The wingtip area is also subject to a vortex, limiting lift.


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Why will the bending of air over an airfoil make a force?

As seen here, the air will bend downwards when it travels over the upper surface. Nothing new here.
The reason it does this is because if the air was not to bend, it would create a vacuum.
The confusing part is why this bending will actually make lift. I understand why the air bends downward over the upper surface, but I don't understand why that would make lift.
A good way to think of it is that air never pulls anything. Only pushes. If that is the case, you could say the air traveling over the upper surface actually gets pushed downwards towards the wing. If you consider it that way, why will there be lift created from the air being bent downward? The air molecules are getting pushed towards the wing's upper surface under their own power, so to speak.
To summarize, I understand why a pressure difference between the surfaces would make lift, but because air can't pull anything I don't understand why the air bending over the upper surface would make lift.

(This might seem like a dumb question, but I've been thinking of it on a molecular level and it doesn't really make sense to me that way)

Imagine yourself on a cart with very low friction wheels on a plain and smooth surface. With you on that cart you have a supply of tennis balls. Or brass ingots, doesn't matter as long as what you have with you has some mass. Now imagine what happens if you start throwing them in one direction.
The cart with you on it would start moving in the opposite direction. By accelerating some mass in one direction, you experience a reaction force in the opposite direction. Or, as Isaac Newton said: Actio equals reactio.
Basically, the pressure field around the wing accelerates an air mass downwards, and the reaction force is applied to the wing as lift. Or, as I am not tiring to repeat that lift can be explained in several ways, the wing accelerates a stream of air downwards, experiencing lift as the reaction force. Or, the more intense bouncing of air molecules against the lower side of the wing and the less intense bouncing on the upper side give them an added impulse downwards. Again, the reaction, summed up over the wing's surface, is lift.

Yes, a thousand times yes.  The flow causes a pressure field, which in turn causes "the air to bend," and causes forces and torques such as lift and drag.  Start with the pressure field.

Sorry for late comment, but just to clarify is the last comment on this answer correct? (The other comments can provide context) Thanks for your answer!

@Wyatt It is all connected. Of course does the pressure difference over an area create a force when summed over that area. But it also causes the flow to change direction (in that case it is better to speak of a pressure gradient - a difference means some jump in value and only happens across a solid surface or a shock). But I think the comment meant something else than what you thought when you spoke of "bending" - the answer itself is very good.

air never pulls anything.  Only pushes.

Not quite right.  On a molecular level, viscosity of moving air does pull in surrounding air.
But in the lift case, you are right.  Air from underneath the wing pushes the wing up into the lower pressure above the wing (created by camber and angle of attack).
The air flow above the wing bends downwards because ambient air is also of higher pressure as compared with the localized lower pressure just above and behind the wing.

Interesting. Regarding the first paragraph of your answer: So the viscosity will pull the other air molecules due to friction I’m guessing?

@Wyatt yes, and the working fluid does not even have to be air.  A common sink aspirator uses moving water (from a faucet) to create a vacuum in a closed tube.

I see. So similar to how if you rub your hand along a piece of paper the paper will get pulled in the direction of movement. As for the rest of your answer: So the bending of air directly above the wing doesn’t really make the lift? Another question I asked recently here would agree with that, so that  would make sense.

The objective of a cambered wing is to tilt the momentum vectors of oncoming air parcels downwards. This requires the wing to apply a force to the air as it passes by and the reaction force is equal to the lift generated by the wing.
There are several different mathematical models of this process, with varying levels of accuracy and complexity, but none of them by themselves captures all the relevant physics. The best models are instead finite-element compressible flow simulation programs that run on large computers, and all the major airplane designers use them.


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What are typical resins used in carbon fiber for aircraft? Are any resins impossible to recycle?
What types of resins are typically used in elements of aircraft, such as structural beams or sheets?
Are some of these commonly used resins more easy to recycle than others? I've that Pyrolysis and Solvolysis are used to recycle the carbon fibers, but it is unclear to me which resins make the carbon fiber composites able to be recycled.

Are you looking for brands or chemistries?

I don't know the specifics of what Boeing or Airbus use, but the vast majority of carbon composites produced use epoxies, which are not easily recycled. However according to wiki they can be essentially pyrolised off in an oxygen-free environment, so the fibres can be recovered at least, though they'd likely have to be downcycled.

Either is fine. I was thinking more along the lines of materials or families of materials. For example, would I want to look in the realm of thermoplastics in general? Or is there a specific epoxy that is widely used for the carbon fiber composites?

This will give you some detail on thermoset resins, which solidify through chemical reaction. In summary:

Epoxy is the primary resin used in aerospace, at least for structures, as it is strong and wear resistant.
Phenolic is the primary non structural resin used in aerospace, as it is fire resistant. This is important in aircraft interiors.
Polyester is cheaper than epoxy but weaker, so it shows up in less structural or weight critical applications. Boats might use this.
Vinyl ester is somewhere in between epoxy and polyester, so not as good as either. I have never used this.
Bismaleimide (BMI) is used for high heat applications where maintaining strength is important. This is a slightly less expensive member of the Polyimide family, used for similar purposes.
Cynate ester apparently has military applications, with which I am not familiar.
PEEK as suggested in comment, is the only thermoplastic in this list, which is formed by melting and solidification as by injection molding. It can be recovered for reuse with reduced mechanical properties.

Thermoset resins are generally not recyclable. When composites are intended for recycling the fibers are the object, not the resin. The resin has to be burnt or dissolved away, creating an emissions problem that needs to be dealt with.

You might want to add polyimide resins and PEEK for high temperature applications. And even the fibers are not fully recyclable; they mostly end up as short fibers for undirected reinforcement, not as woven material or unidirectional tapes.

Currently major structural elements make the most use of bismaleimide and epoxy thermosets. PEEK thermoplastics are gaining share. There's a lot of these and other resins; one of the best-known suppliers with good detail on their website is Cytek.
Thermoplastics can be melted away and are partially recycled for low performance uses. Thermosets are non-recyclable. This, along with lower cost processes, is among the drivers for thermoplastics growth.
Now that composites are the new norm, some expect the next materials advancement metric after composite % to be thermoplastics percentage. The point isn't so much resin recovery as the ease and thus likelihood of recycling.
The recovered fiber is chopped up further and used in bulk in low performance applications. Most consumer goods and aftermarket car parts advertised as carbon only use a layer of woven CF on the outside, with the thickness built up by fiberglass or chopped CF mats with resin matrix.
Short (microns) chopped fiber is also used to fill [thermo]plastics for conventional bulk plastic manufacturing processes. That can be found in cell phone chassis and plastic structural parts in consumer products.


text:
How do you orientate the helicopter at night?
How do you orientate the helicopter at night?

Hi @Toni, and welcome to the Aviation StackExchange! In its current form, your question isn't a good fit for this website. Questions should be clear, well-researched, and directly related to aviation -- this question in particular could use some extra research on your part.

Keep the rotor on top!  Keep visible lights below you. Or be instrument trained and use the gyro's which show the orientation of the helicopter to the terrain.


text:
Why are pressures equal across the slip line of a lambda shock, but density is not?
Why are pressures equal across the slip line of a lambda shock, but density is not? In this answer, it says this :

The surface Σ
is a slip line between zones 3 and 4. Velocities will be parallel across that line. Pressures will be equal across the slip line. Temperature, density, entropy, etc. will be different across the slip line.

So why are the pressures equal across the slip line? I thought low density almost always came with low pressure, and vice versa. In that case, how could you have different density's with the same pressures? Thanks.

As I mentioned in one of my many answers / replies -- think of the slip line like an invisible thin membrane.
Velocity must be parallel because otherwise it would pierce the membrane.
Pressures must be equal because otherwise they would push the membrane up/down.
Everything else (density, temperature, enthalpy, etc.) can be different.
This is compressible flow.  When you go through a shock, you have an abrupt change in properties -- including a jump in entropy.  Therefore Bernoulli's equation does not work across a shock.
When you're considering a lifting flow with shocks, the flow around the upper and lower surface will go through different shocks -- the amount of entropy change will be different for the top and bottom surface flows.  Therefore, Bernoulli's equation can not be used to compare top/bottom flows.
$P=\rho\,R\,T$
If $P$ and $\rho$ are not going hand-in-hand, then it must be because Temperature is different (or you've had a change in gas composition, $R$, but if that has happened, you've also had changes in temperature.)

I see, thanks. So put simply, the shocks change the gas composition/properties, that changes the entropy along with other things, allowing the sides of the slip line to have different density's?

A high temperature shock will change the composition and properties more.  A more mild shock won't change the composition -- but it certainly will add entropy and will cause the temperature to jump.  The entropy change is enough to make Bernoulli not work.  The temperature change shows how P and rho can vary differently.

ah, I understand. Do you know where I could see more about entropy? I don't quite understand how it works.

In the context of compressible flow, you'll want to look at a Gas Dynamics book.  In general, there are three things in a fluid flow that are not isentropic.  1) friction - in the boundary layer, entropy changes.  2) work addition - flow that goes through an engine, propeller, or wind turbine. 3) shocks.  Maybe there are some others, but those three will get you most of the way.

I see. Last thing I'll ask before I read more about that subject : Increases in temperature always comes with an increase in pressure, right? (From what I've read so far, entropy is basically how random the molecules are, or that's what I've interpreted it as) If that's the case, then after the lambda shock, pressure shouldn't be equal on both sides of the slip line even though it is? (because higher temperature comes with higher pressure, and both sides of the slip line experience different temperatures)

"Increases in temperature always comes with an increase in pressure, right?" Wrong. Look at the ideal gas law. There are three other variables in the equation besides P and T.

Both density and temperature are different across the slip line.  Pressure is the only thing equal.  The product of temperature and density must be equal before/after.  You are confused because these ideas are often introduced with Boyle's law and Charles' law and Gay-Lussac's law.  These are simplified versions of the ideal gas law -- but they only apply in special conditions such as constant temperature, constant pressure, or constant volume (say a sealed bottle of gas).  The general solution is the ideal gas law $P=\rho,R,T$ or $P,v=n,R,T$.

Confusion can arise with $P=\rho,R,T$ vs. $P,v=n,R,T$.  Aerospace engineers tend to use the former version, with R being a specific gas constant for air and density being the inverse of specific volume.  Scientists tend to use the latter form with volume and the universal gas constant R, but then they must know how many particles they are dealing with (which is never a real thing).  You can also use a version in terms of specific volume and molecular weight.  These are all the same idea, get comfortable with the one(s) used in your field.

I see, thanks for the modified version of the equation. So in P=pRT, the pressure is P, the density is p, R is the gas constant, and T is temperature. Considering this, that's all pressure relies on. So behind a shockwave, the density is higher, the gas constant stays the same (I think), and the temperature is higher. What I'm getting at is this : What in that equation differs and changes to make the pressure the same above and below the slip line? What values have to be what, if that makes sense. (also I'm probably wrong about some of those things)

The flow went through different shocks. Different shocks with different strengths cause different jumps in properties.  Not all shocks are the same.

Ah I see, makes sense. This is the last little thing I was curious about; I don't want to bore you with questions. : R doesn't stay the same as said in your answer. What is the fundamental thing that makes it change? (definitely my last follow up question, I scrolled up and I've asked a lot of them)

You should read the Wikipedia article for Gas Constant.  R for a specific gas is equal to the universal R divided by the molar mass of the gas.  Of course air is not a single gas, but a mixture of gases.  The average molar mass of air is about 28.964917.  Air is 78% Nitrogen, Nitrogen's molar mass is about 28.  Air is 21% oxygen.  Oxygen's molar mass is about 32.  R will be different when you consider the mixture of combusted gases exiting an engine.  R is different when you consider a helicopter flying in the mostly C02 atmosphere of Mars.

R will also be different in the extreme environment around a spacecraft re-entering the earth.  In particular, in a hypersonic flow, the bow shock around the spacecraft can be so strong that the gas will get so hot that Oxygen $O_2$ and Nitrogen $N_2$ will disassociate into $2O$ and $2N$.  When this happens, the average molar mass changes because there are now twice the number of particles for the same mass.

Oh okay, cool.  (well re-entry isn’t very cool). That Wikipedia article definitely answered my questions, thanks for that. I find it intriguing that so much heat will change the molecular makeup of the air. Thanks for all of your intuition, I greatly appreciate it!

The ideal gas law can actually be derived from theory based on a bunch of particles bouncing around in a box.  The same path will also lead to a derivation of the speed of sound $a=\sqrt{\gamma,R,T}$.  Knowing this helps to explain why changing the number of particles is such a big deal -- 2x the particles, many more collisions, very different behavior.


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Will supersonic flow return to subsonic through shocks over a flat surface?
Will supersonic flow return to subsonic through shocks over a flat surface? Imagine supersonic flow over a plate. Will it always try to return to subsonic through a series of normal shocks?
(Pretend there’s an infinite plate going supersonic)
I’d imagine over an infinite plate, one shock wouldn’t be enough to eliminate supersonic flow over the rest of the surface, and I always thought if there was supersonic flow a normal shock would try to form.

If the shock is perpendicular then yes

@sophit so the flow would turn into a shock? (A perpendicular shock, or normal shock)


text:
Why aren’t flaperons seen on many aircraft?
Why aren’t flaperons common? In theory, they seem like a great idea. I know some planes like the 777 have them, but they’re very small in comparison to the normal flaps.
It would make sense to integrate the flaps and ailerons together, so is there any reason it’s not done more?

Usually a simple solution is best.  Complexity adds weight, cost, maintenance, failure modes, certification challenge, etc.  In aviation, we should always strive for the most simple solution possible -- only adding complexity when it is required.
Start with a simple general aviation aircraft.  It might not even have flaps.  The ailerons are probably mechanically directly controlled by a series of pushrods, bell-cranks, and torque tubes connected to the stick.
When you add flaps (mechanically connected to the flap lever), you add a second system and you take away some of the wing trailing edge area for the ailerons.  You must balance what goes to aileron vs. high lift.
As that aircraft increases in complexity, you likely go to a flap with some sort of fowler action.  For a large transport aircraft, it will be a multi-element fowler flap.
Meanwhile, the aileron will almost always remain a 'plain flap' type -- because it can actuate symmetrically up and down.
When the main high lift system consists of double or triple slotted fowler flaps, the added lift you get from a small amount of plan flap is not much.
Further, now you must actuate the flaperon asymmetrically at some times and symmetrically in othter times.  If these systems are mechanically connected, this means some sort of mechanical mixing.  If they are actuated electrically, hydraulically, or electro-hydraulically, then there will be some level of added complexity.  You might be able to figure out how to do it in hardware, but it may mean that the dreaded software comes into play.  Although it may seem harmless, software is a tremendous challenge in aviation and adds much complexity and cost.
So as with everything else -- a decision is a cost benefit trade.  Is some feature or technology optional or required to meet the desired performance?  Does it 'buy its way on the airplane'?
For most aircraft, flaperons don't meet this standard.

Also, lowering the ailerons will reduce their effectiveness for roll control. Unwise when being tossed about by gusts on final.

A counterexample for when complicated is justified:  some large sailplanes and America's Cup sailboats have so many trailing-edge control surfaces, interacting in so many ways, that traditional nomenclature is abandoned.

In addition to complexity there is also a safety issue.
Ailerons and flaps both change the camber of the wing.  With ailerons, the difference in camber causes the plane to roll.
A malfunctioning flap system (unfortunately) will do the same thing.  This is one of the reasons flaps are tucked in close to the fuselage, and ailerons are further out on the wing.  In the event of a flap malfunction, ailerons are still available for controlling roll.  As a last resort, rudder could also be applied.
A large airliner such as a 787, has a roll control system that uses spoilers, flaps, and ailerons, providing a greater range of roll response and safety in redundancy.
Systems like these are not needed in GA aircraft that fly in a much smaller speed envelope with simple mechanical linkages between the pilot stick/yoke and control surfaces.

A bigger reason for flaps inboard ailerons outboard is simply better control.  Give the ailerons a bigger lever arm, and let the flaps increase washout.

As the name suggests, a flaperon is a control surface that can act both as a flap and as an aileron (actually if deployed upward past the stall angle, it can even work as a spoiler to get a so-called spoileflaperon - and if mounted on a canard surface, it could even work as an elevator and be called elevatospoileflaperon ).

Why aren’t flaperons common

Flaperon are not that common because they are neither fish nor fowl.
If deployed one up and one down then they work just fine like any aileron; and if deployed both downward they work like a simple plain flap. Among all the flaps, a plain flap is the easiest one to be implemented: an hinge is all it takes. Anyway by an aerodynamic its performance is quite limited: it can actually even double the $C_{L_{max}}$ but it does so at the expense of a quite high increase in drag. More complicated flap configurations, like a double or even a triple slotted flap, are more efficient giving an even bigger increase of $C_{L_{max}}$ but with a definitely lower increase in drag.
Mechanically implementing its movement is also complicated since both a coupled and an opposite movement have to be achieved, which needs a complicated mixing unit. And obviously if the flaperons are used as flaps then they cannot be used as ailerons, at least not with the same authority.
For all these reasons their use is limited to all those cases where a low stall speed isn't that important and/or some proper flaps are there to be used when the flaperons act as flaps. Typically they are found on jet fighters and on jetliners where they are used as inboard aileron and as plain flap downstream from the engine exhaust.

Haha what would the name of an elevator, canard, rudder, flap, and aileron be? I think another reason they’re not used too much is because if you just need a small aileron deflection, more drag would be made than normal ailerons, as they’re further out on the wing.


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Are oblique shocks formed at the leading edge-top surface of a wing?
Do oblique shocks form on the leading upper surface of a wing? I know bow shocks are formed there, but is that the same thing as I’m talking about? Oblique shocks would have to form there because the air turns into itself due to the airfoil curve, (at Mach speeds of course). Thanks.

You should know by now that by a pointy body an oblique shock wave (or compression expansion) forms, while by a blunt body a detached shock forms. Those are two different phenomena: behind an oblique shock wave the speed is still supersonic, while behind a detached shock the speed is subsonic. Which of the two is going to happen depends on the geometry of the leading edge.


text:
Why can’t flying wings enter a flat spin?
In this answer, it states:

Naturally stable flying wings never enter a flat spin; their spin modes are all fairly steep due to the lack of a strong inertial moment from the lengthwise distribution of masses.

I don’t think I fully understand what this quote is trying to say.
What part of a flying wing prevents it from flat spinning? If a normal airplane can flat spin, why can’t a flying wing?
In other words, which of the characteristics that differentiate a normal airplane from a flying wing prevent said flying wing from flat spinning?

In order to maintain a flat spin, an airplane with natural longitudinal stability needs a strong pitch-up moment. Without it, the tail will weathervane it into a dive. What provides this pitch-up moment is the lengthwise arrangement of masses which produces a high moment of inertia around the pitch axis.
When spinning, the nose of the fuselage is closer to the axis of rotation, so its masses experience a lower centrifugal acceleration than the masses near the tail, where the rotation radius is much larger. This will pull the parts farther from the axis of rotation more away from this axis than the parts closer to it. The consequence is a pitching moment which pulls the fuselage into a horizontal orientation - just what a flat spin requires.
A flying wing has a small moment of inertia around the pitch axis and, consequently, will experience a much smaller inertial pitch-up moment than an airplane with a fuselage and separate tail surfaces.

You could say the flying wing makes a poor gyroscope.


text:
How does the weak / strong shock solution work?
In this chart, you can see that there are 2 possible angles for a shock to be at when both the deflection angle and Mach number are the same.
Circled in red is the line of an object going Mach 2, and it has a definition angle of 15 degrees.
How is this shock able to have 2 different angles at the same speed and deflection angle?
What is different?

The flow will almost always prefer the weak shock solution.
The strong shock will occur in situations where the back pressure is substantially higher.  This won't happen in an external flow -- because the back pressure will generally eventually be the ambient pressure (the pressure in front of the shock).
So, we're looking at internal flows instead -- inlets to engines.  If the back pressure is too high, the strong shock solution will occur.
You may have heard about 'inlet unstart' occurring with the SR-71 and other aircraft.  This is a situation where the engine is operating normally -- then something occurs that causes the back pressure to increase.  This forces the inlet shocks to switch from the weak to strong shock solution, which then messes up the flow into the engine.
Before they developed a computer to help quickly re-start the inlets, this was a very violent incident and the pilots had to descend and slow down to get the inlets operating properly again.
So there is your next clue to situations like this (where there are multiple solutions).  Sometimes the solution you end up at depends not only on the conditions at that moment -- but also on the history of how you got there.  This property in physics is called hysteresis.
So under normal operation, the inlet sees the weak shock solution -- but once it is unstarted, it sees the strong shock solution.  The fix is to back away from that flight condition until a weak shock solution is obtained and then accelerate back to the initial condition maintaining the weak shock solution.  This is hysteresis in action.

I see, so basically because the shock is stronger it is able to turn the flow the same even at a higher shock angle? (The only reason it is stronger is because it needs to be, due to higher back pressure)

Or, you could look at it as the shock is stronger, therefore the pressure jump is higher.  Importantly, the flow downstream of the strong shock is subsonic -- the flow downstream of the weak shock is supersonic.


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Could ramjets be implemented on sub/super-sonic aircraft wings due to the shockwaves generated by them?
Well, I'm a ignorant on aerodynamics and such, but let me explain first:
In this video (it could be just a video, but) it is said that wings are angled backwards to avoid supersonic shock waves generated by the the shape of the wings.
Therefore, in my ignorant perspective, it seems that this type of occurrence would allow for a ramjet to work on subsonic speeds, since the airflow that would theoretically hit them would already achieve supersonic speeds.
I think ramjets are a really fuel-hunger type of turbine, but nevertheless, I can't stop put wonder if this can be efficiently be applied in any way (even if the wing shape isn't being applied through the entire wing, but in a specific point where the ramjet would be placed).

What do you mean by your third paragraph?  It seems like it contradicts itself - i.e. how could a ramjet work at subsonic speeds if the airflow is supersonic?

@MichaelHall In high subsonic flight, there are portions of the airflow around a wing that can achieve supersonic velocities. So it's not unreasonable to ask if the same could happen at an air intake.

@HiddenWindshield, I know that, and the way you stated it makes complete sense.  But if that is what the OP is saying in this section it isn't worded very well.

Regardless of the shape of the wing, ramjets only outperform turbojets at supersonic airspeed. Ramjets work because the speed of the incoming airflow is such that there is no rotating compressor/turbine required for keeping the jet thrust going. So "I think ramjets are a really fuel-hunger type of turbine.." is a misconception, ramjets have no turbine.

Wikipedia shows how they are constructed, and mentions that they are sometimes called flying stovepipes - the incoming air is slowed down to a slow crawl, and thereby compressed so much that fuel can be injected directly in the high-pressure area to obtain the trust necessary to keep the air flowing into the inlet. Ramjets really work well at speeds between Mach 3 and Mach 6 according to the wiki.

How the incoming air is compressed is important for the efficiency of the cycle. Supersonic shockwaves come with a loss in total pressure $p_t$, with oblique shockwaves yielding smaller losses than a normal shockwave. The pic above (from prof. Wittenberg's course handout that I hung on to) shows a Spike Inlet like in the wiki plot in design state, this one with 1 oblique and 1 normal shock. Pic below shows that a simple pitot inlet has large losses above about M = 1.5, adding more oblique shocks adds efficiency.

Looking at the Wikipedia article on ramjets, we have:

Although ramjets have been run as slow as 45 metres per second (160
km/h), below about Mach 0.5 (170 m/s; 610 km/h) they give little
thrust and are highly inefficient due to their low pressure ratios.

So supersonic speed is not necessary for ramjets to work, although

Due to the stoichiometric combustion temperature, efficiency is
usually good at high speeds (around Mach 2–Mach 3, 680–1,000 m/s,
2,500–3,700 km/h), whereas at low speeds the relatively poor pressure
ratio means the ramjets are outperformed by turbojets, or even
rockets.


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Would a sealed hot air balloon get more lifting at 20km+ of altitude?
So, the record altitude for a manned helium balloon is 40 km/25 miles/131234 feet, while the record altitude for a manned hot air balloon is 21 km/13 miles/68897,6 feet.
My guess is since the hot air balloon is open and it can only heat the air around its altitude, the rarefied atmosphere doesn't allow for a bigger difference in pressure between the inside of the balloon and the outside (not to mention the surrounding temperature).
So, I was wondering: would a sealed hot air balloon would make any difference?

I would suggest to heat the air inside of it electrically instead of using methane, but the question isn't about how it gets hot.
If the envelope was elastic or not and if this would make any difference, I don't know.

A sealed hot air balloon doesn't work at all.
Beyond a certain point, hot air balloon envelopes don't expand any more as the air inside is heated up. Instead, as the air heats up inside, some of the air is actually forced out of the balloon. This makes the balloon lighter and gives it its buoyancy.
If you seal a hot air balloon and try to heat it up, the volume of the balloon will stay mostly the same and so will the density. Instead, the pressure will increase, which doesn't contribute to buoyancy at all.

There are several factors to consider.  The buoyancy of a hot air balloon is achieved by having the air inside it hotter than ambient, and so of lower density.  As the balloon ascends it will expand and so the surface area from which heat is lost goes up.  In any case, the air needs to be heated in flight otherwise it will quickly lose buoyancy.  The ambient temperature initially drops as altitude increases, but at higher altitude this no longer holds.  In general, if the balloon is capable of expanding without resistance and the ratio of absolute temperatures of ambient and internal air is maintained then the buoyancy will be constant.


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Why ionocrafts (ionic propelld aircrafts) don't use electromagnetism to improve performance?
Well, ionocrafts use ion wind in order to propel itself in the air, but that needs a lot of surface area in order to work.
And since ionised gases can be magnetically manipulated, I was wondering if combining ion thrusters with electromagnetic accelerators would work.

However, I couldn't find any kind of information on the subject. Normally the results only talk about ion thrusters or plasma propulsion only.
I guess that the weight would be a problem, but maybe there is a "middle term" where one balances these two systems.

The closest result I could find was this pulsed plasma thruster propulsion system of a satellite that uses Teflon, where the cathode and anode are positioned in such a way that the electrical current helps the plasma a little bit due to its design.
But nothing beyond that.

Does this answer your question?  https://aviation.stackexchange.com/questions/96627/why-don-t-ionocrafts-ionic-propelled-aircrafts-use-ducted-ionic-thrusters-inst?rq=1

@MichaelHall no, that question asks why it is not ducted, not electromagnetically driven.

OK, I thought some of the limitations of such a system that the answer provides might be relevant though.

An ionocraft does use an electromagnetic accelerator. It works by generating an electric field that accelerates the ions between two plates and taking advantage of the interaction between the ions and the air to develop thrust.
Note that this is a distinct mechanism from an ion thruster as used in a spacecraft. Since there is no air to interact with, a spacecraft has to carry the materials to be ionized on board and eject them from the back of the spacecraft in order to develop thrust. This requires a different design of accelerator than an ionocraft, which generates the ions from the surrounding air and then recaptures them.


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What is the G Force graph of the limits of tolerance of humans to linear acceleration in the laying position?
It is not hard to find the limits of tolerance of humans to G forces, but I can't seem to find more detailed graphs for other positions.

This one was taken from wikipedia page about G-Forces, but as shown bellow, supposedly, laying down on different positions can help you withstand more G's for longer.

But I couldn't find more detailed graphs such as the first one, I took this picture from this news article talking about the subject.

As you stated there are no exact plots for human g-force tolerance in laying position readily availlable.
We can, however, use the graph you linked, which is based on work of Dr. Mike Rickards:

The Gemini safe-ejection acceleration criteria we used were based on human tolerance acceleration limits as postulated by Dr. Mike Rickards of Weber Aircraft Corporation and are shown in figure 14

Excerpt from Robert V. Brulle (2008). Engineering the Space Age: A Rocket Scientist Remembers

We can pretty safely assume the "transverse" line in the graph quite well correlates with that of a prone and supine positions. The figure depicts a person sitting down, and at higher transverse (or lateral) g-loadings the normal gravity has only a small effect on g-tolerance of a person. The effect will depend on whether the transverse acceleration is driving blood towards the legs or away from them. The graph is close enough especially when we take into account the vast spread in individual differences when it comes to g-load tolerance.
At high g-loading the tolerance is not simply a matter of maintaining consciousness. Prolonged high acceleration will cause tissue damage, especially in brain tissue, and after passing out, the subject will eventually die if the acceleration keeps the blood pressure in the brain elevated.
On a further note: from the "transverse" line in the figure 14 we can see that the picture in your question depicting pilots in different positions cannot be correct. As per the graph, at best a human body can stand 14 g's shown in the middle position of the picture for several seconds, not minutes.
Note that the time scale in the graph is logarithmic, and by continuing the line we can safely assume that to remain conscious for three minutes, the pilot in the middle position could not pull more than  approximately 6 g's. I dare say at this time the pilots visual cortex would most likely be badly swollem from the pressure of blood, so at least temporary blindness would be imminent.


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Would this concept of Autogyro/Gyroplane be efficient or practical?
First, i'm quite ignorant on aerospace engineering, so I hope I don't make too many incorrect assumptions.
If you don't know what an Autogyro/Gyroplane is, here is the Wikipedia article of it.

The concept:

Basically, the idea would be to have two (or less/more) rotors on opposite sides of the aircraft, however, the shaft of the rotors would be connected to smaller propellers that would be exposed to the jet of air from the aircraft's engine.
The idea would be to take advantage of this jet of air generated by the engines to partially increase the speed of rotation of the rotors, since in the case of a Gyrocopter/Autogyro, they need the aircraft to move in a direction to be able to glide (of course, there are types of Autogyros that are connected to the engine).
I don't think that would be enough to make the aircraft take off vertically, but maybe it could produce more lift for heavier aircraft.
And to be honest, I think this rotor would spin so fast that you would have to add an electromagnetic brake, and well, if you have an electromagnetic brake, you have an electromagnetic motor. What could also change the speed of the rotors to more or less revolutions.
I know if the goal is vertical takeoff, then hook the rotors to the engine right away with a transmission or something. But in my (ignorant) view, this would increase complexity, weight and maintenance costs.
And for the little I know, I can imagine that it wouldn't be that efficient since the thrust would be spent to spin the rotors, but I don't know if this waste would be small or big, or even if it would be enough to make it rotate fast enough to generate any significant lifting power.

Why the question:

I ask this because one of the few VTOL planes that almost reached the civilian market was the "Fairey Rotodyne".
According to what was said about this aircraft, the rotors (which worked as in an Autogyro/Gyrocopter after the vertical take-off) were able to eliminate 40% of the aircraft's total weight during flight; making it an interesting option.
However, the way this hybrid plane took off vertically was pumping fuel to the tip of the propellers (turning it into a Tip-jet), which added to the complexity of the aircraft.
And that was just one of the reasons that it didn't attract many customers: when turning the tips of the propellers into rockets, an absurd noise was generated (and if that noise could be louder than the two engines in the wings, it was certainly something that could not be ignored).

You cite the Fairey Rotodyne -- read the article you linked.  No orders when the prototype was flying, and no significant advantage over more conventional helicopters.

@ZeissIkon Like I said in my post, I'm not refering to the Rotodyne concept, but a totally different concept. The Rotodyne was a VTOL aircraft, I'm suggesting something more akin to a STOL.

Oh, didn't read in high detail, sounded like yet another gyroplane question.  You're asking about rotors driven by ram air turbines, aka windmills in the airstream?

@ZeissIkon kinda off. The Air streams generated by the turbines would increase (in a number I don't know) the speed of rotation of the rotors.

While I’m sure it’s not what you were thinking of, a turbo shaft engine is essentially what you are describing with the propeller you are placing in the exhaust flow optimized into a power turbine.  and those are used to power helicopters.

There's no sensible reason to use ram air turbines to power the rotors on a craft like a gyroplane.  The rotor is already powered directly by the airflow -- essentially "gliding" in and upward flow through the rotor disk.  Adding turbines in the airstream, with shafts and gears to drive the rotors, only inserts inefficiencies -- friction and gear losses, and aerodynamic losses in the turbines.
If you want to shorten the takeoff of a gyroplane, the simplest way to do it (dating back to the 1930s at least, perhaps the 1920s) is a prerotator powered by either a takeoff from the main propulsion engine or a pretty small auxiliary engine or electric motor -- I've seen this done with a spring driven motor -- dedicated to this task.  If you also have collective pitch control on the rotor and enough power in the prerotator, this can even be used for zero-roll "jump" takeoff (the ultimate STOL, no headwind needed) by storing momentum in a slightly overspeed rotor that's converted to lift by a sharp change in collective pitch (flat, to positive to "jump" then back to gyroplane-normal slightly negative to drive autorotation).
A prerotator requires little power; once the takeoff roll begins or some forward speed is developed after the jump, the airstream powers the rotor as with a simple autogyro.